1. Technical Field
The disclosure described herein relates, in general, to an on-demand ink jet head ejecting continuously a plurality of ink droplets to form one pixel on a recording medium. The disclosure specifically relates to an ink jet head having a nozzle plate in which a nozzle having a stepped inner surface is formed.
2. Description of the Related Art
An ink jet head including a plurality of nozzles to eject ink droplets is widely utilized in a home appliance, e.g. ink jet printer. The ink jet head includes a substrate, a spacer formed like a frame adhered to the substrate, and a nozzle plate adhered to the spacer. The substrate, spacer and nozzle plate compose an ink flowing chamber in association with one another. In the ink flowing chamber, a piezoelectric member is provided as an actuator to cause an ink droplet to be ejected from the nozzle plate in which a nozzle is formed.
The piezoelectric member has a plurality of grooves each of which opens towards the nozzle plate. The grooves are arranged in parallel at a predetermined distance. Grooves adjacent to each other are partitioned by a side wall made of the piezoelectric member. A room surrounded by inner surface of each groove and the nozzle plate forms an ink pressure chamber fluidly communicating with the ink flowing chamber. An electrode is deposited on the inner surface of the wall facing the ink pressure chamber.
The nozzle plate has a plurality of nozzles. Each nozzle defines a miniaturized hole in micron size, penetrating the plate. The nozzle is fluidly communicating with the ink pressure chamber in which the electrode is formed on the inner surface thereof.
When a drive voltage is applied to the electrodes, the two side walls faced with each other, interposing one ink pressure chamber, deform in chevron to mutually approach and move apart, for example. Such deformation of the side walls causes ink filled in the one ink pressure chamber to be compressed. As a result, ink in the one ink pressure chamber is ejected out of the nozzle to a recording medium.
Generally the nozzle for ejecting ink is tapered such that diameter of the nozzle gradually decreases in a direction of ink ejection. However it is difficult to drill the tapered nozzle accurately. When a solid material, e.g., silicon, is processed to form a nozzle plate, variation in size of the nozzles tends to occur, even if dry-etching process is utilized to drill the nozzle.
Japanese Laid-open Patent Application No. 2008-87367 (hereinafter called as “JP '367”) discloses an inkjet head having an orifice (nozzle) plate made of silicon. The orifice plate has a nozzle and nozzle communication portion formed in a stepped shape on an inner surface thereof (hereinafter called as “stepped nozzle”). The stepped nozzle is formed in a silicon plate by dry-etching process. The stepped nozzle is of straight hole such that a sectional area of the nozzle is smaller than that of the nozzle communication portion in a plane orthogonal to the direction of ink ejection. The orifice plate is attached to a fluid path substrate such that the nozzle locates at an ink ejection side of the plate and the nozzle communication portion locates at a side opposite to the ink ejection side (fluid path substrate side) in a direction of ink ejection.
In order to form the stepped nozzle, a SOI (Silicon-On-Insulator) substrate (orifice plate) in which a first silicon element, a silicon-dioxide thin film, and a second silicon element are layered in the order is etched from both surfaces thereof.
When the SOI substrate is etched to drill the nozzle and nozzle communication portion, the silicon-dioxide thin film serves as an etching stopper layer. The respective first and second silicon layers are grinded to adjust the thickness of each layer. Therefore, each depth of the nozzle and nozzle communication portion can be independently determined by controlling the amount of grinding the respective surfaces of the SOI substrate.
The shape of the nozzle and nozzle communication portion is realized by photolithographic process, resulting in high precision. For this reason, a variation in size can be suppressed in the stepped nozzle comparing to a tapered nozzle.
Ink jet printer equipped with an inkjet head is required to achieve higher print quality at all times. The print quality is affected by variation of an ink volume ejected from a nozzle of the inkjet head.
In the inkjet head having the stepped nozzle disclosed in JP '367, ink filled in the stepped nozzle is ejected to a recording medium when a drive voltage is applied to the ink jet head. Ink within the nozzle is sharply accelerated every time that the drive voltage is applied to eject ink.
The nozzle of the stepped nozzle serves as a fluid resistance when the accelerated ink is ejected through the nozzle. The fluid resistance depends on length of the nozzle in a direction of ink ejection. Thus, the length of the nozzle significantly affects the ejected ink droplet in its volume.
According to the conventional photolithographic process, an accurate shape of the stepped nozzle can be realized. However, since accuracy in length of the nozzle depends on performance of the machine process in which the surface of the substrate is ground or polished, it is difficult to obtain a highly accurate length of the nozzle. As a result, variation of the length of the nozzles causes an ink volume ejected to fluctuate, resulting in deterioration in a print quality.